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Cisco Security Advisory

Windows VPN Client Local Privilege Escalation Vulnerability

Advisory ID: cisco-sa-20060524-vpnclient

Revision 2.0

For Public Release 2006 May 24 16:00  UTC (GMT)

Related Resources:

View related Alert


The Cisco VPN Client for Windows is affected by a local privilege escalation vulnerability that allows non-privileged users to gain administrative privileges.

A user needs to authenticate and start an interactive Windows session to be able to exploit this vulnerability.

Cisco has made free software available to address this vulnerability for affected customers. There is a workaround available to mitigate the effects of the vulnerability.

This advisory is posted at

Affected Products

This section provides details on affected products.

Vulnerable Products

The following versions of the Cisco VPN Client for Windows (excluding Windows 9x users) are affected:

Products Confirmed Not Vulnerable

All other versions of Cisco VPN Client that are based on an operating system other than Microsoft Windows are not affected by this vulnerability. These include:

  • Any version of the Cisco VPN Client for Solaris
  • Any version of the Cisco VPN Client for Linux
  • Any version of the Cisco VPN Client for Macintosh (MacOS X and MacOS Classic)

Cisco VPN Client for Windows version 4.8.01.x and later are not affected by this vulnerability.

In addition, due to a regression, version of the Cisco VPN Client for Windows is not affected, even though other 4.7.x are affected. Users running version of the Cisco VPN Client for Windows do not need to upgrade to version 4.8.01 or later to be protected from this vulnerability.

No other Cisco products are currently known to be affected by this vulnerability.

Determining the Cisco VPN Client Version

In order to determine which version of the Cisco VPN Client is running on the Microsoft Windows machine, open the Cisco VPN Client graphical user interface by selecting "Programs->Cisco Systems VPN Client->VPN Client" from the Start menu and then select the option "About VPN Client..." from the "Help" menu. This will display a dialog box containing text similar to "Cisco Systems VPN Client Version 4.0.5(Rel)". Please note that the location of the "Cisco Systems VPN Client" folder mentioned above in Windows' Start menu is where the program is installed by default; your system administrator may have chosen to use a different name or location.


The Cisco VPN Client is a software solution for the Sun Solaris, Apple MacOS Classic and MacOS X, Linux and Microsoft Windows operating systems that allows users running these operating systems to establish IPSec VPN tunnels to Cisco VPN-capable devices like Cisco IOS routers, the PIX Security Appliance, the VPN 3000 Series Concentrators, and the ASA 5500 Series Adaptive Security Appliances.

A vulnerability in the Cisco VPN Client for Windows graphical user interface (GUI), also known as the "VPN client dialer", can be exploited to elevate user privileges and obtain LocalSystem-equivalent privileges.

The vulnerability occurs when the Start Before Logon (SBL) feature is enabled on the Cisco VPN Client. This feature can be enabled by unprivileged users from the Cisco VPN Client GUI. Once this feature is enabled, the user will see a Cisco VPN Client GUI in the Windows logon screen. When the user accesses Cisco VPN Client help in this GUI with the F1 key, the Cisco VPN Client spawns a web browser process. Since no user has logged in yet, the Cisco VPN Client is running with LocalSystem privileges, and therefore so will the spawned web browser process. After the logon sequence is completed, the user has a web browser window on the desktop, running with LocalSystem privileges.

Additional information on the SBL feature can be found at:

This issue is not related to any known issues in Microsoft Windows itself.

This vulnerability is documented by Cisco Bug ID CSCsd79265 ( registered customers only)

For information about local system level privileges, please refer to:

Vulnerability Scoring Details

Cisco has provided scores for the vulnerabilities in this advisory based on the Common Vulnerability Scoring System (CVSS). The CVSS scoring in this Security Advisory is done in accordance with CVSS version 2.0.

CVSS is a standards-based scoring method that conveys vulnerability severity and helps determine urgency and priority of response.

Cisco has provided a base and temporal score. Customers can then compute environmental scores to assist in determining the impact of the vulnerability in individual networks.

Cisco has provided an FAQ to answer additional questions regarding CVSS at .

Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks at .


Successful exploitation of the vulnerability may result in a normal user or attacker gaining full control of the system, evading any controls put in place by the Windows system administrator.

Software Versions and Fixes

When considering software upgrades, also consult and any subsequent advisories to determine exposure and a complete upgrade solution.

In all cases, customers should exercise caution to be certain the devices to be upgraded contain sufficient memory and that current hardware and software configurations will continue to be supported properly by the new release. If the information is not clear, contact the Cisco Technical Assistance Center ("TAC") or your contracted maintenance provider for assistance.

This vulnerability is fixed in version of the Cisco VPN Client for Windows, which can be downloaded from the following location: ( registered customers only)

For information on how to upgrade the Cisco VPN Client, including automatic upgrades, please refer to the following chapter of the VPN Client Administrator Guide: Updating VPN Client Software.


A workaround for this vulnerability is to prevent the LocalSystem account (also known as "SYSTEM") from launching the application associated with files with the ".html" extension. After applying this workaround, pressing the F1 key on the Cisco VPN Client GUI in the Windows logon screen has no effect.

This can be accomplished by attaching an Access Control List (ACL) entry to the Windows registry keys HKEY_LOCAL_MACHINE\Software\Classes\.html and HKEY_CLASSES_ROOT\.html so that the NT-AUTHORITY\SYSTEM account is denied all access to that key.

The following example uses the tool RegDACL (Copyright(c) 1999-2001 Frank Heyne Software), available from, to apply the Windows registry ACL mentioned above:

C:\>regdacl HKLM\Software\Classes\.html /DGS:F

RegDACL 5.1 - Permissions Manager for Registry keys for Windows NT 4 and above
Copyright (c) 1999-2001 Frank Heyne Software (
This program is Freeware, use it on your own risk!

Denying F access for predefined group "System"
 - adding new ACCESS DENY entry
 - removing existing entry

C:\>regdacl HKCR\.html /DGS:F

RegDACL 5.1 - Permissions Manager for Registry keys for Windows NT 4 and above
Copyright (c) 1999-2001 Frank Heyne Software (
This program is Freeware, use it on your own risk!

Denying F access for predefined group "System"
 - adding new ACCESS DENY entry
 - removing existing entry


Another tool that can be used to accomplish the same is the Regini.exe utility that comes with several versions of the Windows Resource Kit. For more information on how to use Regini.exe, please visit the following Microsoft Knowledge Base article:

Manually editing the Windows registry with RegEdit.exe will also work, but this method does not scale when the operation needs to be performed on a large number of machines.

While Cisco has tested the above workaround in Cisco's test environment, the effectiveness and impact of any workaround is dependent on each customer's particular environment. Customers electing to apply any workaround (rather than upgrading to an updated version of the Cisco VPN Client) are encouraged to test such workaround thoroughly to ensure, among other things, that it does not negatively impact any other applications that may rely on applicable functionality (e.g., the ability of the LocalSystem account to launch HTML files from any application).

Obtaining Fixed Software

Cisco has made free software available to address this vulnerability for affected customers. Prior to deploying software, customers should consult their maintenance provider or check the software for feature set compatibility and known issues specific to their environment.

Customers may only install and expect support for the feature sets they have purchased. By installing, downloading, accessing or otherwise using such software upgrades, customers agree to be bound by the terms of Cisco's software license terms found at, or as otherwise set forth at Downloads at

Do not contact either "" or "" for software upgrades.

Customers with Service Contracts

Customers with contracts should obtain upgraded software through their regular update channels. For most customers, this means that upgrades should be obtained through the Software Center on Cisco's worldwide website at

Have your product serial number available and provide the URL of this notice as evidence of entitlement to a software patch or bug fix. Customers without service contracts should request a software patch or bug fix through the TAC.

Refer to for additional TAC contact information, including special localized telephone numbers and instructions and e-mail addresses for use in various languages.

Customers Using Third-Party Support Organizations

Customers whose Cisco products are provided or maintained through prior or existing agreement with third-party support organizations such as Cisco Partners, authorized resellers, or service providers should contact that support organization for guidance and assistance with the appropriate course of action in regards to this advisory.

The effectiveness of any workaround or fix is dependent on specific customer situations such as product mix, network topology, traffic behavior, and organizational mission. Due to the variety of affected products and releases, customers should consult with their service provider or support organization to ensure any applied workaround or fix is the most appropriate for use in the intended network before it is deployed.

Customers Without Service Contracts

Customers who purchase direct from Cisco but who do not hold a Cisco service contract and customers who purchase through third-party vendors but are unsuccessful in obtaining fixed software through their point of sale should obtain software patches and bug fixes by contacting the Cisco Technical Assistance Center (TAC). TAC contacts are as follows.

  • +1 800 553 2447 (toll free from within North America)
  • +1 408 526 7209 (toll call from anywhere in the world)
  • e-mail:

Exploitation and Public Announcements

The Cisco PSIRT is not aware of any public announcements or malicious use of the vulnerability described in this advisory.

This vulnerability was independently reported to Cisco by Andrew Christensen from FortConsult and by Johan Ronkainen. Cisco would like to thank them for working with us towards coordinated disclosure of this vulnerability.

Applying an ACL to the HKCR\.html registry key in order to prevent the SYSTEM account from being able to launch a web browser was suggested to Cisco Systems by Johan Ronkainen.

FortConsult's advisory is available at the following location:

Status of This Notice: Final


A stand-alone copy or Paraphrase of the text of this document that omits the distribution URL in the following section is an uncontrolled copy, and may lack important information or contain factual errors.


This advisory is posted on Cisco's worldwide website at

In addition to worldwide web posting, a text version of this notice is clear-signed with the Cisco PSIRT PGP key and is posted to the following e-mail and Usenet news recipients.


Future updates of this advisory, if any, will be placed on Cisco's worldwide website, but may or may not be actively announced on mailing lists or newsgroups. Users concerned about this problem are encouraged to check the above URL for any updates.

Revision History

Revision 2.0


- Added full details about the vulnerability (see the Details section.)

- Provided possible workarounds (see the Workarounds section).

- Added a link to FortConsult's advisory

- Changed "Status of this Notice" from INTERIM to FINAL.

Revision 1.0


Initial public release.

Cisco Security Procedures

Complete information on reporting security vulnerabilities in Cisco products, obtaining assistance with security incidents, and registering to receive security information from Cisco, is available on Cisco's worldwide website at This includes instructions for press inquiries regarding Cisco security notices. All Cisco security advisories are available at

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